I'm looking for examples of companies acting as marketplaces in the sharing economy where users could cheat after the initial transaction, but the company has figured out how to effectively stop it. As an example of the problem: mopp.com. Once you've used the site once and have a cleaner, there's no reason not to pay the cleaner cash directly - it's easier and cuts out the middleman. Other examples include dogvacay and similar. As an example of what I'm *not* talking about - Uber. It's a dual-sided marketplace company within the sharing space, but no one's going to go around it, because it's a one-time transaction. Likewise AirBnb (for the most part, plus they also sort out the international transactions for you).
Here are a couple of marketplaces where the user will always have the option to "shop" outside the system but always comes back because of convinience.
Udemy.com, Tareasplus.com, Clarity.fm, flippa.com, 99designs.com, fiverr.com,